Neural tube defects (NTD) observed in the naturally occurring mutant mouse line Crooked tail (Cd) were recently shown by this laboratory to be associated with a missense mutation in the low density lipoprotein receptor 6 (Lrp6) gene, which encodes a coreceptor acting with Frizzled (Fz) in the wingless (Wnt) pathway. Wnt binding through Lrp6/Fz activates a [unreadable]-catenin dependent signaling cascade (canonical) to affect TCF/LEF driven gene transcription and regulate numerous developmental processes, including neurulation. The Cd mutation results in hyperactive Wnt canonical signaling that cannot be inhibited by Dkk1, an antagonist of the Wnt pathway. While Lrp6 is most closely associated with canonical signaling, Wnt can also work through B-catenin independent routes (non-canonical), such as the planar cell polarity pathway (PCP). Noncanonical pathways affect changes in the cytoskeleton and thereby regulate cell polarity. Recently, PCP genes have been associated with NTDs in mouse and Xenopus. This proposal seeks to determine whether Lrp6 impacts neural tube closure through canonical or non-canonical Wnt signaling pathways. Either result would identify presently unrecognized Lrp6 functions, i.e. the ability of canonical Wnt signaling to regulate neurulation, the ability of Lrp6 to impact non-canonical pathways, or a possible Wnt signaling independent function for Lrp6. The morphological cause of failed neural tube closure in Cd and Lrp6-- mice will be examined using static and dynamic imaging, as well as immunohistochemistry and in situ hybridization. Non-canonical signaling will be examined by the Wnt dependent activation of small GTPases (Rho, Rac1, and Cdc42) in Cd and Lrp6-- mouse embryonic fibroblast (MEF) cells. Genetic crosses of Cd and Lrp6-- mice with mouse lines carrying deletions of other genes in the Wnt signaling pathway (Fz3) and canonical Wnt reporters (BatGal) will test the Wnt dependence of the neurulation defect and identify whether Lrp6 impacts NTDs primarily through the canonical or non-canonical pathway.Relevance to public health: Neural tube defects, such as spina bifida and anencephaly, affect 0.5 to 1 per thousand infants, and complex genetic interactions underlie their occurrence. This proposal will further characterize the function of the Lrp6 gene, which has been associated with neural tube defects, to advance our understanding of its potential action in clinical populations. Since Wnt signaling is also important in postnatal brain development, the knowledge gained from studying Lrp6 in neural tube development will also benefit patients suffering from mental retardation, autism and other behavior and mood disorders.